Last night we were eating dinner and she started making pig noises and when challenged mimicked us by pretending to shovel food in her mouth whilst making snorting and grunting noises
I would have found it very, very hard indeed not to get up and slap her hard around the face. That was absolutely disgusting, disrespectful, contemptuous behaviour on her part and I hope you are going to punish her very, very hard for it. Who the fuck does she think she is? 
When you say she is a healthy weight, whereabouts in the range of healthy weight does she lie? Is she convinced she is fat? Is she on the slightly plump side, or has she always been a string bean?
If she's one of life's natural skinnies who is an 'eat to live' person, not getting especially excited by food and managing to be satiated by relatively small amounts, then she's very lucky indeed.
But she needs to understand that we don't all have the same metabolism or genes and some people aren't as easily satisfied as others. She sounds arrogant, smug, controlling and insufferable.
And I wouldn't discount the beginnings of an eating disorder just yet.